Collector for EUV light source

ABSTRACT

A method and apparatus for debris removal from a reflecting surface of an EUV collector in an EUV light source is disclosed which may comprise the reflecting surface comprises a first material and the debris comprises a second material and/or compounds of the second material, the system and method may comprise a controlled sputtering ion source which may comprise a gas comprising the atoms of the sputtering ion material; and a stimulating mechanism exciting the atoms of the sputtering ion material into an ionized state, the ionized state being selected to have a distribution around a selected energy peak that has a high probability of sputtering the second material and a very low probability of sputtering the first material. The stimulating mechanism may comprise an RF or microwave induction mechanism.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 10/409,254filed Apr. 8, 2003, the disclosure of which is incorporated by referenceherein.

FIELD OF THE INVENTION

The present invention relates to the field of the generation of EUV(soft-x-ray) light for such applications as semiconductor integratedcircuit lithography exposure light sources, and more particularly tolight collectors for such devices.

BACKGROUND OF THE INVENTION

The need for such applications as ever increasingly smaller criticaldimensions for semiconductor integrated circuit manufacturing the needhas arisen to move from the generation of Deep Ultraviolet (“DUV”) lightto Extreme Ultraviolet (“EUV”) light, also referred to as soft-x-raylight. Various proposals exist for apparatus and methods for thegeneration of such light at effective energy levels to enable, e.g.,adequate throughput in an EUV lithography tool (e.g., a stepper scanneror scanner) over an acceptable lifetime between, e.g., replacements ofmajor components.

Proposals exist for generating, e.g., light centered at a wavelength of13.5 nm using, e.g., Lithium which is introduced into and/or irradiatedto form a plasma which excites the lithium atoms to states from whichdecay results in large part in EUV light photons having an energydistribution centered about 13.5 nm. The plasma may be formed by anelectrical discharge using a dense plasma focus electrode in thevicinity of a source of lithium in solid or liquid form, e.g., asdiscussed in U.S. Pat. Nos. 6,586,757, entitled PLASMA FOCUS LIGHTSOURCE WITH ACTIVE BUFFER GAS CONTROL, issued to Melynchuk et al. onJul. 1, 2003, and the above referenced patent application Ser. No.10/409,254 filed Apr. 8, 2003, and U.S. Pat. No. 6,566,668, entitledPLASMA FOCUS LIGHT SOURCE WITH TANDEM ELLIPSOIDAL MIRROR UNITS, issuedto Rauch et al. on May 20, 2003, and U.S. Pat. No. 6,566,667, entitledPLASMA FOCUS LIGHT SOURCE WITH IMPROVED PULSE POWER SYSTEM, issued toPartlo et al on May 20, 2003, which are assigned to the assignee of thepresent application and applications and patents and other referencesreferenced therein, the disclosures of all of which are herebyincorporated by reference, and also other representative patents orpublished applications, e.g., United States Published Application No.2002-0009176A1, entitled X-RAY EXPOSURE APPARATUS, published on Jan. 24,2002, with inventors Amemlya et al. the disclosures of which are herebyincorporated by reference. In addition, as noted in, e.g., patents andpublished applications U.S. Pat. No. 6,285,743, entitled METHOD ANDAPPARATUS FOR SOFT X-RAY GENERATOIN, issued to Kondo et al. on Sep. 4,2001, U.S. Pat. No. 6,493,423, entitled METHOD OF GENERATING EXTREMELYSHORT-WAVE RADIATION . . . , issued to Bisschops on Dec. 10, 2002,United States Published Application 2002-0141536A1 entitled EUV, XUV ANDX-RAY WAVELENGTH SOURCES CREATED FROM LASER PLASMA Published on Oct. 3,2002, with inventor Richardson, U.S. Pat. No. 6,377,651, entitled LASERPLASMA SOURCE FOR EXTREME ULTRAVIOLET LITHOGRAPHY USING WATER DROPLETTARGET, issued to Richardson et al. on Apr. 23, 2002, U.S. Pat. No.6,307,913, entitled SHAPED SOURCE OF X-RAY, EXTREME ULTRAVIOLET ANDULTRAVIOLET RADIATION, issued to Foster et al. on Oct. 23, 2001, thedisclosures of which are hereby incorporated by reference, the plasmamay be induced by irradiating a target, e.g., a droplet of liquid metal,e.g., lithium or a droplet of other material containing a target of, ametal, e.g., lithium within the droplet, in liquid or solid form, with,e.g., a laser focused on the target.

Since the amount of energy in the EUV light desired to be producedwithin the desired bandwidth, from the creation of such a plasma andresultant generation from the plasma of EUV light, is relativelyenormous, e.g., 100 Watts/cm², its is necessary to ensure that theefficiency of the collection of the EUV light be made as high aspossible. It is also required that this efficiency not significantlydeteriorate, i.e., be able to sustain such high efficiency, overrelatively extended periods of operation, e.g., effectively a year ofoperation at very high pulse repetition rates (4 KHz and above) for aneffective 100% duty cycle. Many challenges exist to being able to meetthese goals aspects of which are dealt with in explaining aspects of thepresent invention regarding a collector for an EUV light source.

Some issues that are required to be addressed in a workable designinclude, e.g., Li diffusion into the layers of a multi layer normalangle of incidence reflecting mirror, e.g., through an outer coating ofruthenium (“Ru”), with the multilayered mirror made, e.g., ofalternating layers of Molybdenum (“Mo” or “Moly”) and silicon (“Si”) andthe impact on, e.g., the primary and/or secondary collector lifetime;chemical reactions between, e.g., Li and Si and the impact on, e.g., theprimary and/or secondary collector lifetimes; scatter of out of bandradiation, e.g., from the laser producing the irradiation for ignitionto form the plasma, e.g., 248 nm radiation from an KrF excimer laserrequired to be kept low to avoid any impact on resist exposure giventhat Deep UV resist types may be carried over into the EUV range oflithography and such out of band light scattered from the target canresult in exposing the resist very efficiently; achieving a 100 Wdelivery of output light energy to the intermediate focus; having alifetime of a primary and secondary collector of at least 5 G pulses;achieving the required conversion efficiency with a given source, e.g.,a given target, e.g., a target droplet or target within a droplet, orother targets, the preservation of lifetime of the required multi layermirrors at operational elevated temperatures and out of band radiationat center wavelengths near, e.g., 13.5 nm.

It is well known that that normal incidence of reflection (“NIR”)mirrors can be constructed for wavelengths of interest in EUV, e.g.,between about 5 and 20 nm, e.g., around 11.3 nm or 13.0-13.5 nmutilizing multi-layer reflection. The properties of such mirrors dependupon the composition, number, order, crystallinity, surface roughness,interdiffusion, period and thickness ratio, amount of annealing and thelike for some or all of the layers involved and also, e.g., such thingsas whether or not diffusion barriers are used and what the material andthickness of the barrier layer is and its impact on the composition ofthe layers separated by the barrier layer, as discussed, e.g., in Braun,et al., “Multi-component EUV multi-layer mirrors, Proc. SPIE 5037 (2003)(Braun”); Feigl, et al., “Heat resistance of EUV multi-layer mirrors forlong-time applications,” Microelectronic Engineering 57-58, p. 3-8(2001) (“Feigl”), U.S. Pat. No. 6,396,900, entitled MULTILAYER FILMSWITH SHARP, STABLE INTERFACES FOR USE IN EUV AND SOFT X-RAY APPLICATION,issued to Barbee, Jr. et al. on May 28, 2002, based upon an applicationSer. No. 10/847,744, filed on May 1, 2002 (“Barbee”) and U.S. Pat. No.5,319,695, entitled MULTILAYER FILM REFLECTOR FOR SOFT X-RAYS, issued toItoh et al. on Jun. 7, 1994, based on an application Ser. No. 45,763,filed on Apr. 14, 1993, claiming priority to a Japanese applicationfiled on Apr. 21, 1992 (“Itoh”).

Itoh discusses materials of different X-ray refractive indexes, forexample, silicon (Si) and molybdenum (Mo), alternately deposited on asubstrate to form a multilayer film composed of silicon and molybdenumlayers and a hydrogenated interface layer formed between each pair ofadjacent layers. Barbee discusses a thin layer of a third compound,e.g., boron carbide (B₄C), placed on both interfaces (Mo-on-Si andSi-on-Mo interface). This third layer comprises boron carbide and othercarbon and boron based compounds characterized as having a lowabsorption in EUV wavelengths and soft X-ray wavelengths. Thus, amulti-layer film comprising alternating layers of Mo and Si includes athin interlayer of boron carbide (e.g., B₄C) and/or boron basedcompounds between each layer. The interlayer changes the surface(interface) chemistry, which can result in an increase of thereflectance and increased thermal stability, e.g., for Mo/Si whereinter-diffusion may be prevented or reduced, resulting in these desiredeffects. Barbee also discusses varying the thickness of the third layerfrom the Mo-on-Si interface to the Si-on-Mo interface. Barbee alsodiscusses the fact that typically the sharpness of the Mo-on-Siinterface would be about 2.5 times worse than that of the Si-on-Mointerface; however, due to the deposition of the interlayer of B₄C inthe Mo-on-Si interface, such interface sharpness is comparable to thatof the Si-on-Mo interface. Braun discusses the use of carbon barrierlayers to reduce inter-diffusion at the Mo-Si boundaries to improve thethermal stability and lower internal stress and at the same timeincreasing reflectivity. Braun notes that normally the Mo-Si boundaryforms MoSi₂ at the interface in varying thicknesses at the Mo-on-Siboundary and the Si-on-Mo boundary, and also that the morphology of theMo and/or Si layers can be influenced by barrier layers of, e.g., carboncontent. In addition Braun notes the impact of barrier layer formationon interface roughness of the Mo-Si interface without a barrier layer.Braun reports a reflectance at λ=13.3 nm of 70.1% using Mo/SiCmulti-layers. The reduction in internal stress using B₄C even withannealing as compared to Mo/Si/C multi-layers, which impacts the abilityto uses such multi-layer mirrors for curved mirrors is also discussed.Braun also discusses the tradeoff between interlayer contrast, impactingreflectivity, and absorption in the multi-layer configurations, suchthat, e.g., NbSi layers with lower absorption in the Nb but also lowercontrast, and Ru/Si with higher contrast but also higher absorption inthe Ru layer, both performing less effectively than a Mo/Si multi-layerstack. Braun also discusses the theoretical utility of using threelayers of, e.g., Mo/Si/Ag or Mo/Si/Ru, which have theoretically higherreflectivity, but that the Ag embodiment fails to achieve thetheoretical reflectivity due to voids in the Ag layer at desiredthicknesses and a calculated best reflectivity of a Mo/Si/C/Rumulti-layer stack at λ=13.5 nm, with a thickness constrained in the Molayer to prevent crystallization in the Mo layer. However, Braun alsofinds that the Mo/Si/C/Ru multi-layer stacks do not live up totheoretical calculated reflectivity expectation, probably due to aninitial Mo layer deposition surface roughness that propagates upwardthrough the stack. Feigl discusses the impact of elevated temperaturesup to 500° C. on the structural stability of, e.g., Mo/Si andMo/Mo₂C/Si/Mo₂C multilayer stacks, including the use of ultrathin Mo₂Cbarrier layers. Feigl notes that the barrier layer prevents theformation of inter-diffusion layers of MoSi_(x) due to annealing of theMo and Si at temperatures above, e.g., 200° C. and that Mo/Mo₂C/Si/Mo₂Cand Mo₂C/Si systems remain stable up to 600° C. The former system havingultrathin Mo₂C barrier layers (MoSi₂ is also suggested but not tested)layers and the latter is formed by substituting Mo₂C for Mo in amultilayer system. The reflectivity of the Mo₂C/Si system remained above0.8 through 600° C. according to Feigl, whereas the Mo/Mo₂C/Si/Mo₂Csystem tailed off to slightly less than 0.7 at that temperature, andeven decreased to about 0.7 at 400° C.

Applicants in the present application propose certain other materialsfor barrier layers and other potential improvements to the multi-layerstack for EUV applications.

SUMMARY OF THE INVENTION

A method and apparatus for debris removal from a reflecting surface ofan EUV collector in an EUV light source is disclosed which may comprisethe reflecting surface comprises a first material and the debriscomprises a second material and/or compounds of the second material, thesystem and method may comprise a controlled sputtering ion source whichmay comprise a gas comprising the atoms of the sputtering ion material;and a stimulating mechanism exciting the atoms of the sputtering ionmaterial into an ionized state, the ionized state being selected to havea distribution around a selected energy peak that has a high probabilityof sputtering the second material and a very low probability ofsputtering the first material. The stimulating mechanism may comprise anRF or microwave induction mechanism. The gas is maintained at a pressurethat in part determines the selected energy peak and the stimulatingmechanism may create an influx of ions of the sputtering ion materialthat creates a sputter density of atoms of the second material from thereflector surface that equals or exceeds the influx rate of the plasmadebris atoms of the second material. A sputtering rate may be selectedfor a given desired life of the reflecting surface. The reflectingsurface may be capped. The collector may comprise an elliptical mirrorand a debris shield which may comprise radially extending channels. Thefirst material may be molybdenum, the second lithium and the ionmaterial may be helium. The system may have a heater to evaporate thesecond material from the reflecting surface. The stimulating mechanismmay be connected to the reflecting surface between ignition times. Thereflecting surface may have barrier layers. The collector may be aspherical mirror in combination with grazing angle of incidencereflector shells, which may act as a spectral filter by selection of thelayer material for multi-layer stacks on the reflector shells. Thesputtering can be in combination with heating, the latter removing thelithium and the former removing compounds of lithium, and the sputteringmay be by ions produced in the plasma rather than excited gas atoms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an overall broad conception for alaser-produced plasma EUV light source according to an aspect of thepresent invention;

FIG. 1A shows schematically the operation of the system controlleraccording to an aspect of an embodiment of the present invention;

FIG. 2A shows a side view of an embodiment of an EUV light collectoraccording to an aspect of the present invention looking from anirradiation ignition point toward an embodiment of a collector accordingto an embodiment of the present invention;

FIG. 2B shows a cross-sectional view of the embodiment of FIG. 2A alongthe lines 2B in FIG. 2A;

FIG. 3 shows an alternative embodiment of a normal angle of incidencecollector according to an aspect of the present invention;

FIG. 4 shows a schematic view of a normal angle of incidence collectordebris management system according to an aspect of the presentinvention;

FIGS. 5 a-c show a timing of the provision of a collector cleaningsignal/current at RF and/or DC to the collector mirror according to anaspect of an embodiment of the present invention;

FIGS. 6 a and b show schematic views in cross section of aspects ofembodiments of the present invention relating to grazing angle ofincidence collectors;

FIG. 7 shows a plot of grazing angle of incidence reflectivity for avariety of reflective surfaces at given wavelengths of relevance at anangle of incidence of 5 degrees;

FIG. 8 shows a plot of grazing angles of incidence reflectivity for avariety of reflective surfaces at given wavelengths of relevance for 15degrees;

FIG. 9 shows a schematic view of an alternative embodiment of acollector according to an aspect of the present invention;

FIG. 10 shows a calculated number of lithium atoms per droplet vs.droplet diameter, useful in illustrating an aspect of an embodiment ofthe present invention;

FIG. 11 shows a calculated influx of lithium atoms onto a mirror surfacevs. mirror radius useful in illustrating an aspect of an embodiment ofthe present invention;

FIG. 12 shows a calculated required lithium thickness sputter rate vs.mirror diameter useful in illustrating an aspect of an embodiment of thepresent invention.

FIG. 13 shows a required ratio of molybdenum sputter rate to lithiumsputter rate vs. mirror radius in order to have a 1-year life with a 300pair multi-layer coated mirror useful in illustrating an aspect of anembodiment of the present invention;

FIG. 14 shows sputter yield for lithium, silicon, and molybdenum withhelium ions useful in illustrating an aspect of an embodiment of thepresent invention;

FIG. 15 shows normalized helium ion energy along with sputter yields forlithium, silicon, and molybdenum useful in illustrating an aspect of anembodiment of the present invention;

FIG. 16 shows helium ion current density along with the sputter yield oflithium, silicon, and molybdenum useful in illustrating an aspect of anembodiment of the present invention;

FIG. 17 shows total helium ion sputter rates for lithium, silicon, andmolybdenum useful in illustrating an aspect of an embodiment of thepresent invention;

FIG. 18 shows normalized lithium ion energy along with sputter yieldsfor lithium and molybdenum useful in illustrating an aspect of anembodiment of the present invention;

FIG. 19 shows a radiated power density vs. temperature for a black bodyuseful in illustrating an aspect of an embodiment of the presentinvention;

FIG. 20 shows a schematic view of an aspect of an embodiment of thepresent invention;

FIGS. 21 A and B show results of experiments regarding the stoppingpower of helium and argon buffer gases against both tin and lithium ionsaccording to aspects of an embodiment of the present invention; and,

FIGS. 22A-E show results of further examination of the stopping power ofhelium and argon buffer gases against both lithium and tin according toaspects of an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to FIG. 1 there is shown a schematic view of an overallbroad conception for an EUV light source, e.g., a laser produced plasmaEUV light source 20 according to an aspect of the present invention. Thelight source 20 may contain a pulsed laser system 22, e.g., a gasdischarge laser, e.g., an excimer gas discharge laser, e.g., a KrF orArF laser operating at high power and high pulse repetition rate and maybe a MOPA configured laser system, e.g., as shown in U.S. Pat. Nos.6,625,191, 6,549,551, and 6,567,450. The laser may also be, e.g., asolid state laser, e.g., a YAG laser. The light source 20 may alsoinclude a target delivery system 24, e.g., delivering targets in theform of liquid droplets, solid particles or solid particles containedwithin liquid droplets. The targets may be delivered by the targetdelivery system 24, e.g., into the interior of a chamber 26 to anirradiation site 28, otherwise known as an ignition site or the sight ofthe fire ball. Embodiments of the target delivery system 24 aredescribed in more detail below.

Laser pulses delivered from the pulsed laser system 22 along a laseroptical axis 55 through a window (not shown) in the chamber 26 to theirradiation site, suitably focused, as discussed in more detail below incoordination with the arrival of a target produced by the targetdelivery system 24 to create an ignition or fire ball that forms anx-ray (or soft x-ray (EUV) releasing plasma, having certaincharacteristics, including wavelength of the x-ray light produced, typeand amount of debris released from the plasma during or after ignition,according to the material of the target.

The light source may also include a collector 30. e.g., a reflector,e.g., in the form of a truncated ellipse, with an aperture for the laserlight to enter to the ignition site 28. Embodiments of the collectorsystem are described in more detail below. The collector 30 may be,e.g., an elliptical mirror that has a first focus at the ignition site28 and a second focus at the so-called intermediate point 40 (alsocalled the intermediate focus 40) where the EUV light is output from thelight source and input to, e.g., an integrated circuit lithography tool(not shown). The system 20 may also include a target position detectionsystem 42. The pulsed system 22 may include, e.g., a masteroscillator-power amplifier (“MOPA”) configured dual chambered gasdischarge laser system having, e.g., an oscillator laser system 44 andan amplifier laser system 48, with, e.g., a magnetic reactor-switchedpulse compression and timing circuit 50 for the oscillator laser system44 and a magnetic reactor-switched pulse compression and timing circuit52 for the amplifier laser system 48, along with a pulse power timingmonitoring system 54 for the oscillator laser system 44 and a is pulsepower timing monitoring system 56 for the amplifier laser system 48. Thepulse power system may include power for creating laser output from,e.g., a YAG laser. The system 20 may also include an EUV light sourcecontroller system 60, which may also include, e.g., a target positiondetection feedback system 62 and a firing control system 65, along with,e.g., a laser beam positioning system 66.

The target position detection system may include a plurality of dropletimagers 70, 72 and 74 that provide input relative to the position of atarget droplet, e.g., relative to the ignition site and provide theseinputs to the target position detection feedback system, which can,e.g., compute a target position and trajectory, from which a targeterror cam be computed, if not on a droplet by droplet basis then onaverage, which is then provided as an input to the system controller 60,which can, e.g., provide a laser position and direction correctionsignal, e.g., to the laser beam positioning system 66 that the laserbeam positioning system can use, e.g., to control the position anddirection of he laser position and direction changer 68, e.g., to changethe focus point of the laser beam to a different ignition point 28.

The imager 72 may, e.g., be aimed along an imaging line 75, e.g.,aligned with a desired trajectory path of a target droplet 94 from thetarget delivery mechanism 92 to the desired ignition site 28 and theimagers 74 and 76 may, e.g., be aimed along intersecting imaging lines76 and 78 that intersect, e.g., alone the desired trajectory path atsome point 80 along the path before the desired ignition site 28.

The target delivery control system 90, in response to a signal from thesystem controller 60 may, e.g., modify the release point of the targetdroplets 94 as released by the target delivery mechanism 92 to correctfor errors in the target droplets arriving at the desired ignition site28.

An EUV light source detector 100 at or near the intermediate focus 40may also provide feedback to the system controller 60 that can be, e.g.,indicative of the errors in such things as the timing and focus of thelaser pulses to properly intercept the target droplets in the rightplace and time for effective and efficient LPP EUV light production.

Turning now to FIG. 1A there is shown schematically further details of acontroller system 60 and the associated monitoring and control systems,62, 64 and 66 as shown in FIG. 1. The controller may receive, e.g., aplurality of position signal 134, 136 a trajectory signal 136 from thetarget position detection feedback system, e.g., correlated to a systemclock signal provided by a system clock 116 to the system componentsover a clock bus 115. The controller 60 may have a pre-arrival trackingand timing system 110 which can, e.g., compute the actual position ofthe target at some point in system time and a target trajectorycomputation system 112, which can, e.g., compute the actual trajectoryof a target drop at some system time, and an irradiation site temporaland spatial error computation system 114, that can, e.g., compute atemporal and a spatial error signal compared to some desired point inspace and time for ignition to occur.

The controller 60 may then, e.g., provide the temporal error signal 140to the firing control system 64 and the spatial error signal 138 to thelaser beam positioning system 66. The firing control system may computeand provide to a resonance charger portion 118 of the oscillator laser44 magnetic reactor-switched pulse compression and timing circuit 50 aresonant charger initiation signal 122 and may provide, e.g., to aresonance charger portion 120 of the PA magnetic reactor-switched pulsecompression and timing circuit 52 a resonant charger initiation signal,which may both be the same signal, and may provide to a compressioncircuit portion 126 of the oscillator laser 44 magnetic reactor-switchedpulse compression and timing circuit 50 a trigger signal 130 and to acompression circuit portion 128 of the amplifier laser system 48magnetic reactor-switched pulse compression and timing circuit 52 atrigger signal 132, which may not be the same signal and may be computedin part from the temporal error signal 140 and from inputs from thelight out detection apparatus 54 and 56, respectively for the oscillatorlaser system and the amplifier laser system.

The spatial error signal may be provided to the laser beam position anddirection control system 66, which may provide, e.g., a firing pointsignal and a line of sight signal to the laser bean positioner whichmay, e.g. position the laser to change the focus point for the ignitionsite 28 by changing either or both of the position of the output of thelaser system amplifier laser 48 at time of fire and the aiming directionof the laser output beam.

Turning now to FIGS. 2A and 2B there is shown, respectively a schematicview side view of a collector 30 looking into the collector mirror 150,and a cross-sectional view of the rotationally symmetric collectormirror 150 arrangement along cross-sectional lines 2B in FIG. 2A(although the cross-sectional view would be the same along any radialaxis in FIG. 2A.

As shown in FIG. 2A the elliptical collection mirror 150 is circular incross section looking at the mirror, which may be the cross-section atthe greatest extension of the mirror, which is shown in FIG. 1A to bealmost to the focus point 28 of the elliptical mirror 150, so as not toblock target droplets 94 from reaching the ignition point designed to beat the focus point 28. It will be understood, however, that the mirrormay extend further towards the intermediate focus, with a suitable holein the mirror (not shown) to allow passage of the target droplets to thefocus point. The elliptical mirror may also have an aperture 152, e.g.,shown to be circular in FIG. 2A, to allow entry of the LPP laser beam154, e.g., focused through focusing optics 156, through the mirror 150to the ignition point 28 desired to be at the focus of the ellipticalmirror. The aperture 152 can also be, e.g., more tailored to the beamprofile, e.g., generally rectangular, within the requirements, if any ofmodifying the beam optical path to make corrections of the focus of thelaser beam 154 on an ignition site, depending upon the type of controlsystem employed.

Also shown in FIGS. 2A and 2B is a debris shield 180 according to anaspect of an embodiment of the present invention. The debris shield 180may be made up of a plurality of thin plates 182, made, e.g., of thinfoils of molybdenum, extending radially outward from the desiredignition site and defining narrow planar radially extending channels 184through the debris shield 180. The illustration of FIG. 2A is veryschematic and not to scale and in reality the channels are as thin ascan possibly be made. Preferably the foil plates 182 can be made to beeven thinner than the channels 184, to block as little of the x-raylight emitted from the plasma formed by ignition of a target droplet 94by the laser beam 155 focused on the ignition site 28.

Seen in cross section in FIG. 2B, the functioning of the channels 182 inthe debris shield 180 can be seen. A single radial channel is seen inFIG. 2B and the same would be seen in any section of the collector 30through the rotationally symmetric axis of rotation of the collectormirror 150 and debris shield 180 within a channel of the debris shield180. Each ray 190 of EUV light (and other light energy) emitted from theignition site 28 traveling radially outward from the ignition site 28will pass through a respective channel 182 in the debris shield 180,which as shown in FIG. 2B may, if desired, extend all the way to thecollection mirror 150 reflective surface. Upon striking the surface ofthe elliptical mirror 150, at any angle of incidence, the ray 190 willbe reflected back within the same channel 180 as a reflected ray 192focused on the intermediate focus 40 shown in FIG. 1.

Turning now to FIG. 3 there is shown an alternative embodiment accordingto an aspect of an embodiment of the present invention. In thisembodiment, the debris shield 180 is not shown for simplicity and thisembodiment can be utilized with or without a debris shield asappropriate, as discussed in more detail below, as can also, e.g., thesingle elliptical collector mirror shown in FIGS. 2A and B. In thisembodiment a secondary collector reflecting mirror 200 has been added,which may comprise, e.g., a section of a spherical mirror 202, having acenter at the ignition site 28, i.e., the focus of the elliptical mirror150, and with an aperture 210 for the passage of the light from thecollector mirror 150 to the intermediate focus 40 (shown in FIG. 1). Thecollector mirror 150 functions as discussed above in regard to FIGS. 2Aand 2B with respect to rays 190 emitted from the ignition point 28toward the collector mirror 150. Rays of light 204 emitted from theignition site 28 away from the collector mirror 150 which strike thesection of the spherical mirror 202, will be reflected back through thefocus of the elliptical collector mirror 132 and, pass on to theelliptical collector mirror 150 as if emitted from the focus 28 of theelliptical mirror 150, and, therefore also be focused to theintermediate focus 40. It will be apparent that this will occur with orwithout the presence of the debris shield 180 as described in relationto FIGS. 2A and 2B.

Turning now to FIG. 4 there is shown schematically another aspect ofdebris management according to an embodiment of the present invention.FIG. 4 shows a collector mirror 150 connected to a source of current,e.g., DC voltage source 220. This current can be, e.g., one embodimentof the present invention in which the current maintains the reflector ata selected temperature to, e.g., evaporate deposited lithium. Analternate concept for lithium removal from the first collector mirror isto employ helium ion or hydrogen ion sputtering. The low mass of theseions, when kept at low energies (<50 eV), e.g., can lead to extremelylow sputter yield for, e.g., the molybdenum layer and/or the siliconlayer, e.g., in an EUV multi-layer mirror fabricated with Mo/Si layers.

Turning now to FIG. 4 there is shown a debris cleaning arrangementaccording to an aspect of an embodiment of the present invention. Asshown in FIG. 4, a source of current, e.g., DC voltage source 220 may,e.g., be connected to the collector mirror 150, e.g., to a metal, e.g.,aluminum or nickel backing (not shown) for the mirror 150. The mirror150 may thus be heated to an elevated temperature above that of thesurrounding gas, e.g., helium gas, making up the content of the EUVlight source chamber 26 interior. Other heating of the reflector mayoccur according to alternative embodiments of the invention, e.g., byradiant heating from, e.g., a heat lamp (not shown) in the vessel 26.

Another aspect of debris cleaning may incorporate, e.g., as shown inFIG. 4, e.g., the introduction of RF, e.g., from a source of RFfrequency voltage 230 and an antenna, shown schematically at 232 withinthe chamber 26 in FIG. 4. In fact, the RF, as with the DC shown in FIG.4, may be connected to the mirror 150 or a metallic backing (not shown)in which event a dark shield (not shown) made of a suitable conductivematerial and connected to ground potential may be formed over the backof the collector mirror 150, separated from the mirror 150 by aninsulator, e.g., an air gap, and the potential, e.g., DC from DC source220 that is connected also to the mirror 150.

As shown in FIGS. 5 a-c, for a given periodic LPP ignition at times t₁,t₂, t₃, the RF may be replaced by a DC voltage during the time when theignitions occur at t₁, t₂, t₃, and for a short time on either side ofthe ignition time, with RF between such times, at least directly afterthe ignition, if not completely through the next occurrence of the DCpotential during the next ignition. Also shown is that the DC fromsource 220 may be a positive potential during the time of the respectiveignition, perhaps coextensive with the continuous voltage from the RFsource 230, and a negative potential between such positive pulses.

The voltage applied to the collector mirror 150 is meant to, on the onehand, evaporate metallic debris, e.g., lithium emitted from the plasmaduring and after ignition of a target droplet of such lithium or othertarget metallic material. Also evaporated could be metallic elementssuch as K, Fe, Na or the like that appear due, e.g., to impurities inthe lithium target droplets themselves and are similarly deposited onthe collector mirror 150 surface after ignition.

The RF is meant to form a localized ionic plasma, e.g., of excited Heatoms in the vicinity of the collector mirror 150 surface, with theintent that these excited ions in the localized plasma may strikelithium atoms or compounds of lithium on the collector mirror 150 andsputter them off of the mirror surface. This embodiment of the inventioncontemplates, e.g., a balancing between the evaporation mechanism andthe sputtering mechanism, e.g., if the RF is at <500 W power (at 13.65MHz, as dictated by federal regulations for RF frequency sputtering)then the mirror temperature should be maintained at or near some desiredtemperature and if the RF is increased, e.g., to >500 W at 13.65 MHzthen the temperature can correspondingly be reduced.

Turning now to FIGS. 6A and B there are shown aspects of embodiments ofthe present invention relating to alternative collector arrangements. Asshown in FIGS. 6A and B a collector 225 may be composed of, e.g., aplurality of nested shells, forming, e.g., different sections made up,e.g., of elliptical and parabolic reflecting shells, e.g., in FIG. 6 aparabolic shells 230 and 240 and elliptical shells 250 and 260. Theelliptical shells, e.g., 230 and 240 may be comprised of respectivelyfirst parabolic reflecting surfaces 233, 242, and second parabolicreflecting surfaces 234, 244. The elliptical sections 250 and 260 may becomprised of, e.g., elliptical reflecting surfaces 252 and 262. In FIG.6B there is shown an alternative embodiment with an additional twoparabolic shell sections 232 and 236, with section 232 comprising, e.g.,a first parabolic reflecting surface 231 and a second parabolicreflecting surface 234, and section 236 comprising, e.g., a firstparabolic reflecting surface 237, a second parabolic reflecting surface238 and a third parabolic reflecting surface 239.

Each of the reflecting shells 230, 240, 250 and 260 are arranged toreflect between them 100 percent of the light emitted from the ignitionpoint 21 within a section of a sphere from 11° to 55° from an axis ofrotation 310 generally aligned with the focus of the collector 225reflecting shells, with the shells 230, 240, 250 and 260being generallysymmetric about this axis of rotation 310 also. By way of example, theembodiment of FIG. 6A shows an embodiment where essentially all of thelight in the portion of the sphere just described enters at least one ofthe shells 230, 240, 250 and 260. In the case of parabolic shellsections 230 and 240 is incident on the first reflecting surfaces 233,242, and either reflected towards the intermediate focus 40, or is thealso reflected off of the respective second reflective surface 234, 244to the intermediate focus. In the case of the elliptical shell sections250, 260 all of the light entering each such shell 250, 260 is reflectedto the intermediate focus, e.g., because the ellipses formed by thereflecting surfaces 252, 262 each have a first focus at the ignitionpoint 28 and a second focus at the intermediate focus 40.

Depending on the material of the respective reflecting surfaces 233,234, 242, 244, 252 and 262, the angle of incidence of the particularrays, the number of reflections in a given shell section 230, 240, 250and 260, a certain average efficiency of reflection will occur and alsodepending on the construction of the shells a certain percentage of theavailable light will enter each section 230, 240, 250 and 260, suchthat, as illustrated in FIG. 6A 19% is reflected and focused in shellsection 230 at an average total efficiency of 65%, 17% is reflected andfocused in shell section 240 at an average total efficiency of 75%, 43%is reflected in shell section 250 at an average total efficiency of 80%and 21% is reflected and focused in shell section 260 with an averagetotal efficiency of 91%.

FIG. 6B shows an alternative embodiment adding two more parabolic shellsections 232, 236. These added sections may serve, e.g., to collect morelight up to about 85% from the axis of rotation and at least one of theadded sections may have a first reflective surface 237, a secondreflective surface 238 and a third reflective surface 239. As can beseen from FIG. 6B, e.g., a ray 290 of emitted light from the source orignition point may just enter the parabolic reflective shell section 236and be reflected as ray 292 to the second reflective surface 238 andthen reflected as ray 294 to the third reflective surface 239 and thenform focused ray 296. Similarly a ray 300 may just enter the parabolicreflector shell 236 at the other extremity of the shall opening and alsobe reflected off of the first reflective surface 237 as ray 320 and thesecond reflective surface 237 as ray 304 and the very end of the thirdreflective surface as focused ray 306. In the case of, e.g., one of theparabolic shell sections, e.g., section 240 a ray 280 may just enterthis section 240 and be reflected off of the first parabolic reflectingsurface 242 as ray 282 and the very end of the second parabolicreflecting surface as focused ray 283, and another ray 284 may justenter the section 240 to be reflected off of the second reflectivesurface 244 as focused ray 286. In the case of one of the ellipticalshell sections, e.g., 250, a ray 308 emitted from the ignition point mayjust enter the shell section 250 and be reflected off of the ellipticalreflecting surface 252 as a focused ray 309, and a ray 318 just enterthe shell section 250 at the opposite side as ray 308 and be reflectedas focused ray 319.

Turning now to FIGS. 7 and 8 there is shown a plot of grazing angle ofincidence reflectivity for (1) a single layer Ruthenium reflectingsurface and (2) a Mo/Si bilayer stack with a Mo/Si 14 nm thick single Molayer and a 4 nm single Si layer and (3) a ten period multi-layer Mo/Sistack, with a pitch of 9.4 nm and a MO/Si thickness ratio of 22.5:1,having, e.g., 40 multi-layer stacks, each for grazing angles ofincidence of 5° and 15°. In each of the reflectors with Mo/Si amolybdenum substrate is assumed. In the case where spectral purity is apart of the specification for the delivered light, collectors can betuned to a certain wavelength, with some given bandwidth spread, e.g.,by using the reflective properties of, e.g., a nested shell collector tofavor reflectivity near a selected center wavelength, e.g., in theembodiments of FIG. 6A and B.

FIG. 9 shows aspects of an embodiment of the present invention. In thisembodiment a collector assembly 330 may comprise, e.g., a portion of aspherical mirror reflecting surface 332, which may be a normal angle ofincidence multi-layer stack, reflecting the light produced from theignition point 28 to one of, e.g., three nested elliptical shellsections 336, 338 and 340 in a nested elliptical shell collector 334.Each of the shell sections 336, 338 and 340 may have a reflectingsurface 366, 368, 369 that is on the inside of a respective shell 360,362, 364. As shown in FIG. 9, e.g., the shell section 336 may receivethe light from a rim section 370 of the spherical mirror 332, the shellsection 338 may receive the light from an intermediate section of thespherical mirror 332 and the shell section 340 receives the lightreflected from a central portion of the spherical mirror 332.

The shell sections 336,338 and 340 may be coated with a multi-layer ofMo/Si rather than the conventionally proposed thick single layer of Ru.According to aspects of an embodiment of the present invention tworeflections occur, e.g., one from the spherical mirror and one in eachshell, e.g., for shells having elliptical reflecting surfaces, atgrazing angles between about 5° and 15°, as can be seen from FIGS. 7 and8. This can, e.g., significantly reduce, e.g., a significant amount ofout of band EUV radiation, e.g., assuming that 13.5 is the desired band.Ru mirrors, e.g., in a Wolter-type configuration remain very reflectivefor both 13.5 nm and 11 nm at both 5° and 15° grazing angles ofincidence, whereas Mo/Si stacks of grazing angle of incidence reflectivecoatings, as shown in FIGS. 7 and 8 can be much more selective,especially around 15°.

The above described embodiment does not have the spatial purity of,e.g., a grating spectral purity filter, as has been proposed in the art,but it does have a significant advantage in reflectivity andpreservation of in-band EUV radiation over the other solutions, e.g., agrating filter, proposed in the art.

A lithium LPP EUV light source according to aspects of embodiments ofthe present invention, could employ a solid stream of liquid lithium ora lithium droplet source. For a droplet source, the number of atoms perdroplet can be calculated and for a solid stream one can assume thatonly material within the focused beam constitutes a droplet at ignition,although, from a debris standpoint adjacent material in the stream mayalso form debris, particularly if struck by lower energy laser radiationin the skirts of the energy distribution of the focused laser beam.

Since it is contemplated that it is desirable for the droplet source tohave a droplet size matched to the focused beam, both types of targetsource can be considered to have the same droplet size given by adroplet diameter, d_(droplet). The volume of the droplet is then givenby: $\begin{matrix}{V_{droplet} = {\frac{1}{6}\pi\quad{d_{droplet}^{3}.}}} & \left\{ 1 \right\}\end{matrix}$Calculating the number of atoms per droplet follows from the density of,e.g., lithium and its atomic weight. The mass of the droplet is:M_(droplet)=V_(droplet)ρlithium   {2};where ρ_(lithium)=0.535 g/cm³ is the density of lithium, such that:M_(droplet)=0.280·d_(droplet) ³   {3};where the droplet diameter is in centimeters and the resulting mass isin grams. The number of atoms in the droplet is then given by dividingthe droplet mass by the atomic mass of lithium and converting unitsproperly: $\begin{matrix}{{N_{atoms} = {\frac{M_{droplet}(g)}{M_{{lithium}\quad{atom}}({amu})} \cdot \frac{1\quad{amu}}{{1.6605 \times 10^{- 24}}g}}};} & \left\{ 4 \right\}\end{matrix}$where M_(lithium atom)=6.941 amu, i.e.,N _(atoms)=2.43×10²² ·d _(droplet) ³   {5};where the diameter of the droplet is in centimeters. Converting thedroplet diameter from centimeters to micrometers gives:N_(atoms)=2.43×10¹⁰ ·d _(droplet) ³   {6}.

The number of atoms per droplet versus droplet size is shown in FIG. 10Also shown in FIG. 10 is the number of 13.5 nm photons contained in,e.g., a single 40 mj pulse. The 40 mj pulse example assumes a 10%conversion efficiency into 4π steradians and a 400 mj laser pulse. Thenumber of 13.5 nm photons per pulse is given by: $\begin{matrix}{{N_{Photons} = \frac{13.5{nm}\quad{{OpticalPulseEnergy}({mJ})}}{{{E_{Photon}({eV})} \cdot {1.6 \times 10^{- 16}}}\left( {{mJ}\text{/}{eV}} \right)}};} & \left\{ 7 \right\}\end{matrix}$where the 13.5 nm photon energy is 91.6 eV. The resulting number ofphotons for a 40 mj pulse is 2.72×10¹⁵. A, e.g., 50 μm droplet has onelithium atom for every 13.5 nm photon. Normally one could assumemultiple photons emitted from each emission element. This assumptionwould allow use of a smaller droplet diameter than 50 μm. A smallerdroplet diameter can be important because the lithium usage and lithiumdeposition rates, e.g., on the collector optics, scale as the cube ofthe droplet diameter.

Assuming that there is no lithium recovery, according to a possibleaspect of an embodiment of the present invention, then calculating,e.g., the yearly usage of lithium is given by the number of pulses peryear times the amount per pulse. Assuming, by way of example arepetition rate, RR, and a duty cycle, DC, the resulting mass usage is,e.g.,:Mass Per Year=M _(droplet) ·RR·60 sec/min·60 min/hr·24 hr/day·365day/yr·DC   {8}.i.e.,Mass Per Year=8.83×10⁻⁶ ·d _(droplet) ³ ·RR·DC   {9};where the droplet diameter is in micrometers and the resulting mass isin grams. For example, a system with no lithium recovery running at 6kHz with a droplet diameter of 50 μm running at 100% duty cycle for afull year would consume 6,622 grams or about a 12.3 liter volume oflithium. A droplet diameter of 25 μm under similar conditions wouldconsume only 828 grams or about 1.5 liters of lithium.

Assuming that the lithium droplet, once heated by the laser pulse,expands in all directions uniformly, the atomic flux will fall off asthe square of the distance from the laser-droplet interaction point(ignition site). The number of atoms emitted from the interaction pointper second is the number of atoms per droplet times the repetition rate:Total Atomic Emission=2.43×10¹⁰ ·d _(droplet) ³ ·RR   {10};where the droplet diameter is in micrometers and RR is the laserrepetition rate in Hz.

The atomic flux (atoms/cm²) through the surface an imaginary spherecentered at the ignition site will be the total atomic emission dividedby the surface area in centimeters: $\begin{matrix}{{{{Atomic}\quad{Flux}} = {{1.93 \times 10^{9}}\frac{d_{droplet}^{3} \cdot {RR}}{r_{sphere}^{2}}}};} & \left\{ 11 \right\}\end{matrix}$The resulting flux is in units of atoms/cm² s. FIG. 11 shows the rate oflithium influx onto the mirror surface vs. mirror radius for severaldroplet diameters, i.e., (1) 25 μm. (2) 55 μm, (3) 100 μm and (4) 200μm, assuming a 6 kHz repetition rate and 100% duty cycle.

In order to maintain high mirror reflectivity, the influx of lithiumonto the mirror surface can, e.g., be exceeded by the sputter rate oflithium, e.g., caused by incident helium ions. In addition, for longmirror lifetime the sputter rate of molybdenum by these same, e.g.,helium ions must then be many orders of magnitude slower than that for,e.g., lithium.

The required ratio of sputter rate of the first and second metals, e.g.,molybdenum to lithium, in order to achieve, e.g., a I year lifetime forthe multi-layer coated collector mirror can be calculated, e.g., byassuming use of, e.g., a multi-layer stack with 300 layer pairs, e.g.,so that erosion of, e.g., the first 200 layer pairs leaves a stillcomfortably effective 100 good pairs, i.e., still maintaining highreflectivity. Also assumed is a sputter rate for the silicon layers thatis much higher than that for the first metal, e.g., molybdenum layersand thus provides a negligible contribution to the mirror lifetime.

A typical EUV mirror can consist, e.g., of a layer pair of molybdenumand silicon with the molybdenum layer 2.76 nm thick, such that 200 pairsfor sacrificial erosion gives, e.g., 552 nm of molybdenum erosion beforeend-of-life for this mirror. For a 1-year useful life, the molybdenumsputter rate must be below 552 nm/year, i.e., 1.75×10⁻⁵ nm/sec.

The lithium sputter rate in terms of atoms per cm² per second (equal tothe lithium influx rate derived above) converts to nm/sec from thethickness of a monolayer of lithium, given the atomic number density oflithium per its mass density and atomic weight, with appropriate unitconversions, as follows: $\begin{matrix}{{{{Atomic}\quad{Number}\quad{Density}} = \frac{\rho_{lithium}\left( {g\text{/}{cm}^{3}} \right)}{{M_{{lithium}\quad{atom}}({amu})} \cdot \frac{{1.6605 \times 10^{- 24}}g}{1\quad{amu}}}};} & \left\{ 12 \right\}\end{matrix}$where ρ_(lithium)=0.535 g/cm³ and M_(lithium atom)=6.941 amu. Theresulting atomic number density for lithium is 4.64×10²² atoms/cm³. Ifthis number of lithium atoms where arranged in a cube with dimensions 1cm on each side, then the number of atoms along an edge per cm would bethe cube root of the atomic number density, 3.58×10⁷ atoms/cm. Theresulting monolayer thickness is 2.78×10⁻⁸ cm or 0.278 nm. The number ofatoms per cm² in a monolayer then is the square of the number of atomsalong an edge per cm: 1.28 ×10¹⁵ atoms/cm².

The number atoms of, e.g., lithium, removed by sputtering per secondmust match the influx rate given in Equation 11. Thus, the number ofmonolayers removed per second is equal to the influx rate divided by thenumber of atoms per cm² in a monolayer. Thickness removal rate is themonolayer removal rate times the thickness of a monolayer, i.e.,$\begin{matrix}{{{ThicknessRemoval}\quad{Rate}} = {{{MonolayerThickness}({nm})} \cdot {\frac{{InfluxRate}\left( {{atoms}\text{/}{cm}^{2}s} \right)}{{Numberof}\quad{Atoms}\quad{{inaMonolayer}\left( {{atoms}\text{/}{cm}^{2}} \right)}}.}}} & \left\{ 13 \right\}\end{matrix}$Using the values for lithium:LithiumThicknessRemovalRate=2.17 ×10⁻¹⁶·LithiumInfluxRate(atoms/cm² s)  {14}with the resulting units of nm/sec. The lithium influx rate shown inFIG. 11 converts to a required lithium thickness sputter rate, shown inFIG. 12, for the same 1-4 droplet sizes, repetition rate and duty cycle

This result further highlights the need for a small droplet size and alarge mirror radius. Otherwise, the required sputter rate can becomeimpractical.

The required thickness sputter rate for lithium, can be compared to themaximum allowed thickness sputter rate for molybdenum, e.g., for a 1year collector lifetime. The data in FIG. 12 divided into the maximumallowed molybdenum sputter rate, 1.75×10⁻⁵ nm/sec is shown in FIG. 13for the same 1-4 droplet sizes, repetition rate and duty cycle

The question is what is needed to create a molybdenum sputter rate 4 ormore orders of magnitude less than the lithium sputter rate. The sputteryield for lithium and molybdenum when attacked by helium ions isdiscussed, e.g., in W. Eckstein, “Calculated Sputtering, Reflection andRange Values”, [citation to publication?] ______, Jun. 24, 2002. Thissputter yield data versus ion energy is shown in FIG. 14 along with datafor silicon for ion energies of (3) lithium into Mo at Eth=52.7 eV, (2)helium into Si at Eth=10.1 eV and (1) helium into Li. As one can see, aproperly chosen helium ion energy will result in acceptable lithiumsputter yield and essentially no molybdenum sputter yield. A problem canarise, however, from the fact that one cannot control the incident ionenergy perfectly. That is, the energy spectrum of incident helium ionsis not a delta function. It is the spread of ion energies that must beassessed when determining the deferential sputtering between lithium andmolybdenum.

There are examples in the literature of RF Induction (RFI) plasmas whichcreate, e.g., an ion energy distribution that is Gaussian shaped with,e.g., a FWHM of 2.5 eV as discussed, e.g., in J. Hopwood, “IonBombardment Energy Distributions in a Radio Frequency Induction Plasma,”Applied Physics Letters, Vol 62, No. 9 (Mar. 1, 1993), pp 940-942.

The peak of the ion energy distribution can, e.g., be adjusted withproper choice of, e.g., electric field strength and helium pressure. Bychoosing, e.g., a peak ion energy of 20 eV, the helium ions have highsputter yield for lithium, but have energies safely below that of themolybdenum sputter threshold. In FIG. 15 there is shown a plot ofnormalized ion energy distribution (1 on the log scale and 2 on thelinear scale) centered on 20 eV and FWHM of 2.5 eV along with thesputter yields for (3) lithium, (4) silicon, and (5) molybdenum. One cansee that there are very few helium ions with energy above the molybdenumsputter threshold. To determine the sputter rate of molybdenum underthese conditions requires calculating the influx of helium ions neededto maintain the mirror surface clean of lithium atoms. A constantsputter yield of 0.2 atoms per ion can be assumed, since the bulk of thedistribution of helium ion energies falls within the region of nearlyconstant lithium sputter yield. $\begin{matrix}{{{Helium}\quad{{Ionflux}\left( {{ions}\text{/}{cm}^{2}s} \right)}} = {\frac{{Lithium}\quad{{Influx}\left( {{atoms}\text{/}{cm}^{2}s} \right)}}{{Sputter}\quad{{Yeild}\left( {{atoms}\text{/}{ion}} \right)}}.}} & \left\{ 15 \right\}\end{matrix}$Thus, the helium ion density must be 5 times the value of lithium influxdensity shown for various conditions in FIG. 11.

This helium ion influx expressed in Equation 15 may be considered to bethe bare minimum, assuming, e.g., the lithium does not deposit perfectlyuniformly. In this event a higher total sputter rate may be required,e.g., to ensure that islands of lithium do not develop. On the otherhand, other researchers have shown that the ejection of material from anLPP plasma tends to travel toward the laser source. One can, therefore,e.g., arrange the system such that the laser illuminates the lithiumdroplet from a direction away from the collector, or through an aperturein the collector mirror that causes much of this debris to not strikethe collector mirror. Thus, the total lithium load on the mirror may bereduced from the total theoretical amount striking the mirror.

Knowing the total flux of helium ions and assuming a Gausian energydistribution with a peak at 20 eV and a FWHM of 2.5 eV, the integral ofa normalized Gaussian distribution is √{square root over (2πσ²)} whereσ² gives a variance of the distribution related to the FWHM by:$\begin{matrix}{\sigma^{2} = {\frac{({FWHM})^{2}}{4\quad{\ln(4)}}.}} & \left\{ 16 \right\}\end{matrix}$The integral of a normalized Guassian then is$\sqrt{\frac{{\pi({FWHM})}^{2}}{2\quad{\ln(4)}}},$so that the peak current density of helium ions is given by:$\begin{matrix}{{{PeakHeliumCurrentDensty}\left( {{ions}\text{/}{cm}^{2}s\quad{per}\quad{eV}} \right)} = {\frac{{HeliumIonInflux}\left( {{ions}\text{/}{cm}^{2}s} \right)}{\sqrt{\frac{{\pi({FWHM})}^{2}}{2\quad{\ln(4)}}}}.}} & \left\{ 17 \right\}\end{matrix}$Taking the case of a 25 μm droplet with a mirror radius of 10 cm, thepeak helium current density must be 3.38×10⁻¹⁵ ions/cm² s per eV inorder to sputter a total of 1.88×10¹⁵ lithium atoms/cm² s. This heliumcurrent density distribution (1) is plotted in FIG. 16 on a log scale,with (2) silicon sputter density and (3) lithium sputter density, alongwith empirically determined sputter yield of (4) lithium, (5) silicon,and (6) molybdenum and the product of these functions times the ioncurrent density. A surprisingly beneficial result of this analysis showsthat the peak sputter density for molybdenum is 3.5×10⁻²⁰⁵ atoms/cm² sper eV(not shown on graph), an incredibly small value. In fact, even thepeak silicon sputter density is more than 3 orders of magnitude smallerthan that for lithium.

The integral of these sputter densities over all helium ion energiesgives the total sputter rate. These integrals are shown respectively asdashed curves (1) for lithium and (2) for silicon, in FIG. 17. Theintegrated lithium sputter density is 1.88×10¹⁵ atoms/cm² s, matchingthe lithium influx rate. The integrated silicon sputter density is9.17×10¹⁰ atoms/cm² s. The integrated molybdenum sputter density is1.16×10⁻²⁰⁵ atoms/cm² s. Therefore, differential sputter rates betweenmolybdenum and lithium are so low that, e.g., less layers of thecollector mirror need be employed, e.g., many less that a previouslyanticipated 300 base pair mirror concept. A single molybdenum layer willlast more than a year under these conditions and the assumptions of thissputter yield model. This performance could be even more improved usinga debris shield between the ignition spot and the collector main mirroror main an secondary mirrors, but the debris shield, as seen from theseresults, may also be totally eliminated, at least for a lithium target.This type of stimulated plasma induced ionized sputtering of debris fromthe EUV optics, especially for a lithium target, as seen from the above,could even allow for use of other target types, e.g., a moving tape orother type of moving solid target system. Helium ion sputtering can bearranged such that it removes the lithium atoms from the collectormirror at a sufficient rate while sputtering molybdenum at a low enoughrate for far greater than 1 year lifetime.

Sputtering of molybdenum by, e.g., lithium ions must also be consideredin the embodiment of the present invention being discussed, since, e.g.,there will be lithium ions formed a debris from the ignition plasmawhich do not reach the optic surface, but which will be available to thesputtering plasma and will be accelerated toward the mirror surface witha similar energy distribution as the helium ions. The literature alsoprovides data on sputter yield of lithium and molybdenum with lithiumions. This data is shown in FIG. 18 in curve 1 for lithium at Eth=36.3eV, along with the same normalized lithium ion energy distribution aswas used for the helium ions. To calculate the molybdenum sputterdensity from lithium the total lithium ion influx must be known. Unlikethis calculation for helium (Equation 15) it is not clear what the totallithium influx will be, however, a conservative choice would the totallithium atomic influx generated by the LPP ignition plasma. UsingEquation 17 and the assumptions of a 25 μm droplet and a 10 cm mirrorradius, 1.88×10¹⁵ lithium ions/cm² s would be incident on the mirror,and the peak lithium ion current density is 7.06×10¹⁵ lithium ions/cm² sper eV, with the assumption of a 2.5 eV FWHM spread in incident ionenergy, which, when multiplied by the sputter yield for molybdenum andintegrated over all ion energies, gives a total molybdenum sputterdensity of 2.54×10⁻⁴⁸ atoms/cm² s. This is much higher than that forhelium ions, but still much, much lower than the rate required for oneyear of useful life.

The molybdenum sputter density with lithium ions can be converted tothickness loss rate by using Equations 12 and 13. For molybdenum:ρ_(moly)=10.2 g/cm³M _(moly atom)=95.94 amu=1.59×10⁻²² gMoly Atomic Number Density=6.40×10²² atoms/cm³Moly Monolayer Thickness=2.50×10⁻⁸ cm=0.250 nmMoly Monolayer Atomic Density=1.59×10¹⁵ atoms/cm²

Thus, the sputter thickness rate for molybdenum, when attacked bylithium atoms, is 3.99 ×10⁻⁶⁴ nm/sec or 1.25×10⁻⁵⁶ nm/year. This alsoleads to the conclusion that the above noted beneficial results of thesputtering plasma ionized cleaning of the EUV optics by, e.g., heliumion sputtering are still realizable even with, e.g., lithium sputteringof molybdenum.

An additional beneficial result is the reconsideration of the previouslyproposed use of, e.g., a ruthenium capping layer on, e.g., themulti-layer mirror. A ruthenium capping layer has been proposed toprevent EUV-assisted oxidation of the first silicon layer in the Mo/Sistack. Multi-layer mirrors are usually terminated with a silicon layerrather than a molybdenum layer because the molybdenum layer wouldquickly oxidize once exposed to room air. Applicants, before the aboveanalysis regarding sputtering plasma cleaning of the EUV optics hadconsidered, e.g., a multi-layer mirror terminated with silicon, with theexpectation that the first layer of silicon would be eroded to exposethe first layer of molybdenum or a ruthenium capping layer to avoidoxidation of a first layer of molybdenum if that approach was taken. Thesuper-slow erosion rate of molybdenum, and a similar expected lowerosion rate for ruthenium allows for use of a ruthenium capping layerexpected to last for the useful life of the mirror. This results in noloss of the first layer of silicon, and no need to worry about whathavoc the sputtered silicon atoms might cause, and no oxidation problemswith an exposed molybdenum layer. The sputter yield of ruthenium withlithium and helium, although expected to be similar to that ofmolybdenum, since ruthenium has a higher atomic mass than molybdenum,remains to be determined.

The minimum RF power needed to create the desired sputtering plasma ator near the optic surface can be calculated by assuming, e.g., thatevery helium ion that is created strikes the collector mirror, whichwill underestimate the required RF power, but should give an order ofmagnitude estimate. Each helium ion that strikes the collector mirrorrequires 24.5 eV to ionize, and according to the above example of anembodiment of the present invention has to have an average kineticenergy of 20 eV when it reaches the collector mirror. These two energyvalues times the required influx of helium ions, 9.40×10¹⁵ ions/cm² sfrom Equation 15, gives the plasma power. Converting energy units fromeV to J gives a minimum plasma power density of 66.9 mW/cm². Multiplyingby the half the surface area of the 10 cm radius mirror, 628 cm², gives42 W of minimum total plasma power. Assuming conservatively that only 1%of the plasma power is effectively used, the required plasma powercalculated is 4.2 kW, which is acceptable, especially considering thevery large area over which this power can be dissipated. This estimateof plasma power compares to the previous assumptions of 400 mJ per pulseat 6 kHz LPP laser power, 2.4 kW of laser power and assuming thecollector mirror subtends π steradians, it will be exposed to half ofthis laser power, i.e., 1.2 kW. The thermal load from the LPP is similarto the thermal load of the plasma cleaning. The sum of the two powers is5.4 kW, resulting in a power density on the mirror of 8.6 W/cm².Applicants believe that a collector mirror exposed to a 10 W/cm² or lesspower density is easily cooled, e.g., with water channels along the backof the mirror, or between the grounded shield and the mirror.

If the plasma power effectivity is more like 10%, then the total powerdensity onto the mirror is only 2.6 W/cm², making it possible toradiatively cool the mirror, according to Stefan's law of radiation,which states that the power radiated per square meter from a black bodyat temperature T is given by:P=5.67×10⁻¹² ·T ⁴   {18};where temperature is in Kelvin and the resulting power density is inW/cm², which is plotted in FIG. 19. A temperature in excess of 500° C.would be required to radiate all of this incident power, so activecooling of the collection mirror appears to be required in order toprevent damage to the multi-layer stack.

Turning now to FIG. 20 there is shown an schematically an apparatus andmethod according to an embodiment of the present invention forreclaiming damaged EUV optics, e.g., those that have lost reflectivity,e.g., due to deposition of material on the reflective surface, e.g.,carbon and/or carbon based molecules, which may come from, e.g.,contamination entering the EUV plasma chamber or from sputtering orphotonic removal from layers of the multi-layer reflective stack coatedon reflecting surfaces un the EUV apparatus. As can be seen in FIG. 20 aphoto-chemical cleaning apparatus 400 may include a chamber, withinwhich may be mounted, e.g., a collector holding jig 402 that is adaptedto hold a collector for cleaning. Also included may be, e.g., a sourceof photonic energy, e.g., a DUV light source 410, with the collectorholding jig 402 and the light source 410 arranged so that the light fromthe light source 410 simulates light coming from a point source at thefocus of the collector, e.g., the ignition site 28 discussed above, suchthat the collector 404 is irradiated as if by light from a targetignition site.

According to an embodiment of the present invention, e.g., the chamber401 may first be purged by the use of nitrogen provided to the chamberthrough N₂ valve and then evacuated from the chamber 401 using gas exitvalve, followed by the introduction of a fluorine containing gas, e.g.,molecular F₂ or NF₃. The collector 404 may then be subjected toirradiation by the light source, e.g., DUV light at a range of λbetween, e.g., 160-300 nm, e.g., from a KrF excimer laser at 193 nm,e.g., in a MOPA configuration for high power at about 40 W, with a pulserepetition rate at about 4 kHz. This can serve, e.g., to stimulate theproduction of, e.g., fluorine based carbon materials, e.g., CF₄, e.g.,in a gas phase, which can then be evacuated from the chamber 401 throughthe gas exit valve 420 under a second nitrogen purge.

An alternative of a KrF DUV light source could be, e.g., a commerciallyavailable DUV lamp, e.g., a KrCl DUV lamp.

Applicants expect that thicknesses of about 3.5 nm carbon atomdeposition on an EUV optic, e.g., a collector reflective surface canreduce reflectivity by about 5% and a 10 nm deposition by about 14%.Such levels of thickness of deposit are expected to be removed from,e.g., the collector optics reflective surfaces under treatment influorine with selected concentrations and the above referenced level ofDUV light for a selected time. The process could also employreplenishing the fluorine supply with a gas flow control valve (notshown) to maintain, e.g., a desired concentration of fluorine during thecleaning process.

Applicants herein also propose according to an aspect of an embodimentof the present invention that other types of barrier materials may beused in the multi-layer reflecting mirror stacks to help improve thethermal stability and reflectivity of, e.g., Mo/Si reflective stacks,e.g., optimized for 13.5 nm EUV light reflectivity. To promotesmoothness of very thin, e.g., 1 nm barrier layers, that are compatiblewith, e.g., Mo/Si and perhaps also MoSi₂, retaining the appropriatelevels of transparency to, e.g., 13.5 nm light, applicants propose theuse of inter-diffusion barrier layers comprising carbides selected fromthe group comprising ZrC, NbC, SiC, borides, e.g., selected from thegroup ZrB₂, NbB₂, disilicides selected from the group comprising ZrSi₂,NbSi₂ and nitrides BN, ZrN, NbN and Si₃N₄. Other such layers couldinclude yttrium, scandium, strontium compounds and/or these metals inpure form. Among the above, the carbides and borides mentioned arepreferred due to the ability to create smoother diffusion barrier layerswith such materials.

According to aspects of an embodiment of the present inventionapplicants contemplate multi-layer stacks, including e.g., MoSi₂/Si,Mo₂C/Si, Mo/C/Si/C and Mo/X/Si/X, where the first two are MLMs whereMoSi₂ or Mo₂C is used in place of the Mo normally used in normal Mo/Simirror coatings, with no inter-diffusion barriers. The other two arewith the so-called inter-diffusion barriers, where C refers to carbonand X refers to a suitable material, including further compounds, e.g.,the above noted borides, disilicides, and nitrides as the X materials.Nitrides are currently preferred embodiments according to applicants forinter-diffusion barrier layers in the applications according toembodiments of the present invention. Mo₂Si/Si is described in the paperY. Ishii et al. “Heat resistance of Mo/Si, MoSi₂/Si, and Mo₅Si₃/Simultilayer soft x-ray mirrors”, J. Appl. Phys. 78, (1995) p. 5227.

Helium has high transparency to EUV, which makes it a good choice for abuffer gas for which a transmission of 90% is representative. Based onthe partial pressures required for efficient sputtering, a few mTorr,helium buffer gas transmission would be nearly 100%. A possiblecollector multi-layer surface could comprise, e.g., 300 coating pairsinstead of the normal 90 pairs. The extra pairs would not improve thereflectivity over a 90 pair mirror, but instead these extra layers can,if required, get used once the top layers are eroded away. With a 300pair mirror the sputter rate differential between lithium and the mirrorneed not be so high that a single mirror layer lasts fro, e.g., monthsat a time. Instead three could be, e.g., an extra 210 layer pairs worthof mirror erosion that can be sustained.

Lithium chemical compounds that might be generated in the LPP vessel,e.g., LiH, LiOH, Li₂CO₃, etc., can have melting points in excess of 600°C. and thus may not be evaporated from the mirror. These could even formin certain cases, e.g., a crust over the lithium which deposits on themirror surfaces. These could, however, very effectively be sputtered bythe sputtering ion plasma, e.g., containing the ionized He atoms, or maybe sputtered by lithium itself in the form of high speed lithium ionsand atoms ejected from the plasma that impinge on the reflectingsurface.

The sputter rate required to stay ahead of the lithium deposition couldbe much higher in an EUV light source than the literature indicates istypically practiced, e.g., in modern deposition and etch machines, whichis at lease part of a reason for, e.g., a combined approach to keepingthe, e.g., lithium off of the mirror surfaces. According to an aspect ofan embodiment of the present invention applicants contemplate usingevaporation to remove the bulk of the lithium while employing a verylight sputter rate to remove the inevitable lithium and carbon compoundsdeposited on the mirror surface. However, even a very light sputteringplasma impinging on at least the main and secondary reflecting surfacescould have the same beneficial carbon and other lithium compound removalproperties. Employing this idea beyond the intermediate focus, e.g., inthe illuminator reflective surfaces and also the projection reflectingsurface may also prove beneficial to remove debris that happens to reachthe lithography tool reflecting surfaces. In the lithography toolitself, due to, e.g., smaller deposition rates the thermal load andsputtering rate may be sufficiently low for this to be effective.

Sputtered lithium and lithium compounds along with lithium ejected fromthe plasma that does not collect on the reflecting surface may betrapped in cold fingers [not shown] contained in the EUV light sourcevessel, e.g., in the form of cooled, e.g., water cooled fins or platesextending from the inside walls of the vessel, and out of the opticalpath from the collector to the intermediate focus.

In the case of, e.g., tin as the source element it may be possible touse, e.g., a hydrate of the metal, e.g., SnH₄, which is a vapor at roomtemperature, along with a hydrogen plasma for cleaning the collector ina tin-based LPP source. Hydrogen has high 13.5 nm transmission and theresulting SnH₄ could be pumped away rather than trapped on cold fingerslike the lithium.

Applicants have examined, e.g., the stopping power of helium and argonagainst both tin and lithium ions. The results are shown in FIG. 21A andB. The two graphs have the same data, just different scales. Lines 500,502 and 503, for tin at different measured distances from a sourceplasma, respectively 96.5 cm, 61 cm and 32.5 cm with solid being heliumbuffer and dashed being argon buffer. The lines 506 are for lithium. Ifpressure*distance product scaling were applied, these three sets of fortin data would fall approximately on top of each other.

Applicants have also determined that Argon has at least 10 times higherstopping power than helium for a given gas pressure. Also, lithium canbe stopped with less buffer gas than tin. And, scaled to the trueworking distance of an LPP collector (˜10 cm), the required bufferpressure, even with argon, will need to be in the range of about 10 mTfor tin. Since xenon and tin have nearly the same atomic mass,applicants expect that the required buffer pressure for a xenon LPPwould also be in the range of 10 mT. Such a high buffer gas pressure canpresent EUV self-absorption problems for xenon and tin. But not forlithium, both because of the lower buffer pressure requirement and alsothe lower EUV absorption of lithium.

In continuing to examine the stopping power of a buffer against, e.g.,the fast ions produced by the LPP, using, e.g., a Faraday cup to collectand measure the ions at a known distance through a known aperture sizeat different increasing buffer gas pressure this Faraday cup signaldecreased, giving a measure of the ion stopping power. The results fortin and lithium are shown below in FIGS. 22A-E. FIGS. 22A and 22B showthe raw Faraday cup signal vs. time for, respectively tin and lithium.In FIGS. 22C and D these signals are plotted vs. ion energy usingtime-of-flight respectively for tin and lithium. In FIG. 22E the areaunder these curves is plotted vs. a pressure*distance product of thebuffer gas, with the lower plot line (1) being tin and the upper (2)being lithium.

A surprising result of this analysis by applicants was that the lastgraph shows the Faraday cup signal vs. buffer gas P*D product for bothtin and lithium being about the same for both elements. Applicantsbelieve this is explainable in that the analysis was not reallymeasuring the loss of ions captured by the Faraday cup, but instead wasmeasuring the neutralization of the ions by the buffer gas, so-calledelectron capture by the ions. If an ion is neutralized, it will notregister in the Faraday cup. This can be explained, e.g., because tinmight have a larger electron capture cross-section than lithium,especially considering that the tin ion is highly charged, 7-11 timesionized and the lithium can, at most, be 3 times ionized. The stoppingpower result shown in FIG. 22E can be considered an overestimate of thebuffer gas stopping power in that it can be no better than the valuepredicted by these curves.

Taking the observed values of the stopping power as the upper limit onecan calculate the necessary pressure of argon buffer gas to extend thecollector mirror lifetime to 100B pulses. Starting with the result fromEngineering Test Stand (ETS) built by the EUV LLC, which reported thatone multi-layer mirror pair is eroded for every 15M pulses with a xenonLPP and a collector distance of 12 cm, and assuming that thereflectivity of a multi-layer mirror is not significantly degraded until10 layer pairs are removed, the ETS collector mirror had a lifetime of150M pulses compared to a requirement of 100B pulses. This leads to theconclusion that a reduction of 666× in erosion rate is necessary. On theplot in FIG. 22E a P*D product of approximately 500 mT*cm would berequired to achieve this level of reduction. A working distance of,e.g., 12 cm gives, e.g., a need for an argon pressure of 42 mT. Thisalso can result in the conclusion that lithium is the better targetover, e.g., xenon, since for xenon LPP, a buffer pressure of 42 mT isnot very satisfactory due to the strong EUV absorption of the xenoncaught up in the argon buffer. For lithium, however, this amount ofbuffer pressure is no problem for lithium absorption. Tin may also besatisfactory, depending on, e.g., the vapor pressure and evolution rateof SnH4 from the surface of the collector mirror. A relatively largebuffer gas pressure seems, therefore, to be a requirement, which leadsto the conclusion that xenon is not a good target, tin may be, butlithium appears to be the best.

Applicants have also determined that even if the effectiveness ofheating the collector reflective surfaces is impacted by the fact thatthe material being evaporated needs to have, e.g., a certain thickness,e.g., 50 Å, e.g., about 10 monolayers, before published values of vaporpressure are realized, i.e., the material, e.g., lithium may be harderto evaporate directly off of the surface of the mirror, nevertheless,the transmittance of such a thickness of lithium on the mirror surfacesis about 95%, and about 90%, double-pass, so that such a layer on themirror would not significantly detract from the overall CE, e.g., at13.5 nm. In addition, such a layer of “evaporationless” lithium, mayactually be beneficial in that it may be able to protect the collectormirror from the onslaught of high-speed lithium atoms and ions. Thislithium layer will be sputtered instead of the molybdenum layer of themulti-layer mirror. Xenon, because it is a gas, will not form such aprotective layer and a tin layer, because of its very high EUVabsorption, would only be 52% transmitting.

Given that the sputter yield of lithium against molybdenum is much lessthan the sputter yield of xenon against molybdenum, e.g., for ionenergies around 1 keV (the sputter rates tend to saturate above thisenergy level): Incident Ion Target Material Lithium Xenon Lithium 0.21?? Molybdenum 0.081 1.45xenon will sputter molybdenum at 18 times higher rate than lithium. Thisdifference alone would give a 2.7B pulse collector lifetime withoutchanging anything else. The “evaporationless” steady state thin layer oflithium may provide the remaining 37× reduction in sputter rate. Even ifit does not, the EUV LLC concept of producing a mirror with ˜100 extrasacrificial layer pairs could add a 10× increase in lifetime, e.g., to27B pulses, which combined with the lower erosion from lithium couldgive a collector lifetime of 100B pulses.

Applicants have also examined the effectiveness of electrostaticprotection of the collector mirror. The concept has been proposed in theliterature, i.e., to generate an electric field between the source LPPand the collector mirror such that the energetic ions must climb up apotential well as they travel toward the mirror. This potential well canbe made deep enough that the ions loose all of their kinetic energybefore reaching the mirror. In fact, they are turned around and sentpacking back down the potential well, never reaching the mirror.Applicants have discovered, however, that attempting to do this byrunning an electrical connection through the vessel to the collectormirror was ineffective due to the target bias dropping to near zero uponpulsing the laser, which was determined to be the result of a high peakcurrent required to maintain the bias voltage and the large lead wirerequired, thus dropping all of the voltage along the inductance of thewire. To correct this problem applicants then installed capacitorsinside the vacuum vessel and constructed low inductance buss workbetween ground and the target plate. Inductance was measured by placinga copper sheet up and around the target and attached to ground. Bycharging the capacitors to a low voltage and discharging them bypressing the copper sheet against the target applicants measured theringing voltage waveshape and inferred the inductance. The result was104 nH with a 697 ns half-period discharge waveshape. This dischargeperiod is much longer than the laser pulse and subsequent EUV emissioninitially causing concern whether the bias could be maintained duringthe critical period when the ions are created and leave the plasmaregion (˜20 ns). Applicants determined, however, that such short timescales were unimportant. What is important is, e.g., to maintain orreestablish the target bias, e.g., in a time scale that is, e.g., shortcompared with the travel time of the ions from the target to the mirror.With the present geometries the ion travel time is about 2.5 μs, so acircuit half-period of 0.7 μs should be sufficient.

In testing this arrangement applicants were surprised to find that thefull 0.47 μF capacitance was drained of its −1000V potential in a timescale almost exactly the same as during the inductance measurement usingthe copper strap. Applicants determined that the laser pulse initiates adischarge between the target plate and the vessel wall. This dischargecompletes the circuit between the capacitor's high voltage terminal andground, thus draining the capacitors as if a copper strap had beenplaced across them. Evidently, the events that unfold during, andimmediately after, the laser pulse, a plasma being created at the targetpoint and this plasma radiating a large amount of hard UV and EUVradiation throughout the vessel. The energy of most of these photons isabove the work function of the metals inside the vessel and thusphotoelectrons are created at all the metal surfaces. These photons arealso energetic enough to ionize any gas atoms that exist in the vessel.In this case argon was used as the buffer gas and it is easily ionizedby the hard UV and EUV radiation produced by the LPP. And finally,electrons and ions are created in the LPP and stream outward into thevolume of the vessel. Except for those ions that are attracted to thebiased target plate. They strike the plate and create secondaryelectrons. Essentially, the creation of a discharge between two metalplates held at a potential between each other occurs as if thearrangement were a laser-triggered discharge switch.

There still is some possibility of making an effective electrostaticrepulsion, but it becomes a bit more complicate and isn't reallyelectrostatic. The idea is to pulse the bias such that it is presentonly after the initial events of the laser pulse. In only a few 100's ofns most of the electrons will have collided with the vessel wall, and ofcourse the radiation will be gone. At this time it might be possible toapply a bias and repel, or attract, the ions away from the collectormirror.

Those skilled in the art will appreciate that the above referencespreferred embodiments of the present invention and aspects thereof arenot meant to be exclusive and other modifications and additions to theabove referenced embodiments may be made without departing from thespirit and scope of the inventions disclosed in the present application.The appended claims, therefore, should not be considered to be limitedto the above disclosed embodiments and aspects but should include withthe scope and spirit of the claims the recited elements and equivalentsof the recited elements. By way of example, other target material andmulti-layer reflective coating metals may have similar relationships asdiscussed above to allow for the continuous cleaning by, e.g.,sputtering, e.g., of ions, e.g., induced by the creation of a sputteringplasma in the vicinity of the optic reflecting surface(s), which ionsmay also be, e.g., other than helium, e.g., H, N or O. Also, e.g., theheating mechanism for the reflecting surface could be a heat lampdirected at the reflective surface. Other such changes and additions maybe appreciated by those skilled in the art.

1. A debris removal system for the removal of plasma produced residuedebris on a reflecting surface of an EUV collector in an EUV lightsource, wherein the reflecting surface comprises a first material andthe residue debris comprises a second material comprising: a controlledsputtering ion source comprising: a gas comprising the atoms of thesputtering ion material; a stimulating mechanism exciting the atoms ofthe sputtering ion material into an ionized state, the ionized statebeing selected to have a distribution around a selected energy peak thathas a high probability of sputtering the second material and a very lowprobability of sputtering the first material.
 2. The apparatus of claim1 further comprising: the stimulating mechanism is an RF or microwaveinduction mechanism.
 3. The apparatus of claim 1 further comprising: thegas is maintained at a pressure that in part determines the selectedenergy peak.
 4. The apparatus of claim 2 further comprising: the gas ismaintained at a pressure that in part determines the selected energypeak.
 5. The apparatus of claim 1 further comprising: the stimulatingmechanism creates an influx of ions of the sputtering ion material thatcreates a sputter density of atoms of the second material from thereflector surface that equals or exceeds the influx rate of the plasmadebris atoms of the second material.
 6. The apparatus of claim 2 furthercomprising: the stimulating mechanism creates an influx of ions of thesputtering ion material that creates a sputter density of atoms of thesecond material from the reflector surface that equals or exceeds theinflux rate of the plasma debris atoms of the second material.
 7. Theapparatus of claim 3 further comprising: the stimulating mechanismcreates an influx of ions of the sputtering ion material that creates asputter density of atoms of the second material from the reflectorsurface that equals or exceeds the influx rate of the plasma debrisatoms of the second material.
 8. The apparatus of claim 4 furthercomprising: the stimulating mechanism creates an influx of ions of thesputtering ion material that creates a sputter density of atoms of thesecond material from the reflector surface that equals or exceeds theinflux rate of the plasma debris atoms of the second material.
 9. Theapparatus of claim 1 further comprising: the reflecting surface is anormal angle of incidence multilayer reflector that is highly reflectiveto EUV light comprising a laminate of layers of the first material andlayers of a third material.
 10. The apparatus of claim 2 furthercomprising: the reflecting surface is a normal angle of incidencemultilayer reflector that is highly reflective to EUV light comprising alaminate of layers of the first material and layers of a third material.11. The apparatus of claim 3 further comprising: the reflecting surfaceis a normal angle of incidence multilayer reflector that is highlyreflective to EUV light comprising a laminate of layers of the firstmaterial and layers of a third material.
 12. The apparatus of claim 4further comprising: the reflecting surface is a normal angle ofincidence multilayer reflector that is highly reflective to EUV lightcomprising a laminate of layers of the first material and layers of athird material.
 13. The apparatus of claim 5 further comprising: thereflecting surface is a normal angle of incidence multilayer reflectorthat is highly reflective to EUV light comprising a laminate of layersof the first material and layers of a third material.
 14. The apparatusof claim 6 further comprising: the reflecting surface is a normal angleof incidence multilayer reflector that is highly reflective to EUV lightcomprising a laminate of layers of the first material and layers of athird material.
 15. The apparatus of claim 7 further comprising: thereflecting surface is a normal angle of incidence multilayer reflectorthat is highly reflective to EUV light comprising a laminate of layersof the first material and layers of a third material.
 16. The apparatusof claim 8 further comprising: the reflecting surface is a normal angleof incidence multilayer reflector that is highly reflective to EUV lightcomprising a laminate of layers of the first material and layers of athird material.
 17. The apparatus of claim 1 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 18. The apparatus of claim 2 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 19. The apparatus of claim 3 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 20. The apparatus of claim 4 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 25. The apparatus of claim 5 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 26. The apparatus of claim 6 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the fist material sustaining such sputtering for greater than aselected lifetime.
 27. The apparatus of claim 7 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 28. The apparatus of claim 8 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 29. The apparatus of claim 9 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 30. The apparatus of claim 10 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 31. The apparatus of claim 11 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 32. The apparatus of claim 12 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 33. The apparatus of claim 13 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 34. The apparatus of claim 14 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 35. The apparatus of claim 15 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 36. The apparatus of claim 16 further comprising: thesputter thickness rate for sputtering of the first material by thesecond material is at or below a rate that will result in a single layerof the first material sustaining such sputtering for greater than aselected lifetime.
 37. The apparatus of claim 17, further comprising:the reflecting surface comprises a capping layer comprising a fourthmaterial selected to have a sputter thickness rate that will alsosustain sputtering by the second material at or below a rate that willresult in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 38. The apparatus of claim 18, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 39. The apparatus of claim 19, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 40. The apparatus of claim 20, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 41. The apparatus of claim 21, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 42. The apparatus of claim 22, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time-and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 43. The apparatus of claim 23 further comprising:the reflecting surface comprises a capping layer comprising a fourthmaterial selected to have a sputter thickness rate that will alsosustain sputtering by the second material at or below a rate that willresult in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 44. The apparatus of claim 24 further comprising:the reflecting surface comprises a capping layer comprising a fourthmaterial selected to have a sputter thickness rate that will alsosustain sputtering by the second material at or below a rate that willresult in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 45. The apparatus of claim 25 further comprising:the reflecting surface comprises a capping layer comprising a fourthmaterial selected to have a sputter thickness rate that will alsosustain sputtering by the second material at or below a rate that willresult in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 46. The apparatus of claim 26, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 47. The apparatus of claim 27, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 48. The apparatus of claim 28, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 49. The apparatus of claim 29, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 50. The apparatus of claim 30, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 51. The apparatus of claim 3 1, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 52. The apparatus of claim 32, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 53. The apparatus of claim 33, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 54. The apparatus of claim 34, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 55. The apparatus of claim 35, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 56. The apparatus of claim 36, furthercomprising: the reflecting surface comprises a capping layer comprisinga fourth material selected to have a sputter thickness rate that willalso sustain sputtering by the second material at or below a rate thatwill result in a single layer of the fourth material sustaining suchsputtering for greater than the selected time and to have more favorableproperties when exposed to ambient or operating environments than thoseof the first material.
 57. The apparatus of claim 1 further comprising:the first material is molybdenum.
 58. The apparatus of claim 2 furthercomprising: the first material is molybdenum.
 59. The apparatus of claim3 further comprising: the first material is molybdenum.
 60. Theapparatus of claim 4 further comprising: the first material ismolybdenum.
 61. The apparatus of claim 5 further comprising: the firstmaterial is molybdenum.
 62. The apparatus of claim 6 further comprising:the first material is molybdenum.
 63. The apparatus of claim 8 furthercomprising: the first material is molybdenum.
 64. The apparatus of claim9 further comprising: the first material is molybdenum.
 65. Theapparatus of claim 1 further comprising: the second material compriseslithium.
 66. The apparatus of claim 2 further comprising: the secondmaterial comprises lithium.
 67. The apparatus of claim 3 furthercomprising: the second material comprises lithium.
 68. The apparatus ofclaim 4 further comprising: the second material comprises lithium. 69.The apparatus of claim 5 further comprising: the second materialcomprises lithium.
 70. The apparatus of claim 6 further comprising: thesecond material comprises lithium.
 71. The apparatus of claim 7 furthercomprising: the second material comprises lithium.
 72. The apparatus ofclaim 8 further comprising: the second material comprises lithium. 73.The apparatus of claim 1 further comprising: the sputtering ion materialcomprises He.
 74. The apparatus of claim 2 further comprising: thesputtering ion material comprises He.
 75. The apparatus of claim 3further comprising: the sputtering ion material comprises He.
 76. Theapparatus of claim 4 further comprising: the sputtering ion materialcomprises He.
 77. The apparatus of claim 5 further comprising: thesputtering ion material comprises He.
 78. The apparatus of claim 6further comprising: the sputtering ion material comprises He.
 79. Theapparatus of claim 7 further comprising: the sputtering ion materialcomprises He.
 80. The apparatus of claim 8 further comprising: thesputtering ion material comprises He.
 81. The apparatus of claim 1further comprising: a heater element operatively coupled to thereflective surface heating the reflective surface independently of thestimulating mechanism and the ambient operating environment of thereflective surface.
 82. The apparatus of claim 2 further comprising: aheater element operatively coupled to the reflective surface heating thereflective surface independently of the stimulating mechanism and theambient operating environment of the reflective surface.
 83. Theapparatus of claim 3 further comprising: a heater element operativelycoupled to the reflective surface heating the reflective surfaceindependently of the stimulating mechanism and the ambient operatingenvironment of the reflective surface.
 84. The apparatus of claim 4further comprising: a heater element operatively coupled to thereflective surface heating the reflective surface independently of thestimulating mechanism and the ambient operating environment of thereflective surface.
 85. The apparatus of claim 5 further comprising: aheater element operatively coupled to the reflective surface heating thereflective surface independently of the stimulating mechanism and theambient operating environment of the reflective surface.
 86. Theapparatus of claim 6 further comprising: a heater element operativelycoupled to the reflective surface heating the reflective surfaceindependently of the stimulating mechanism and the ambient operatingenvironment of the reflective surface.
 87. The apparatus of claim 7further comprising: a heater element operatively coupled to thereflective surface heating the reflective surface independently of thestimulating mechanism and the ambient operating environment of thereflective surface.
 88. The apparatus of claim 8 further comprising: aheater element operatively coupled to the reflective surface heating thereflective surface independently of the stimulating mechanism and theambient operating environment of the reflective surface.
 89. Theapparatus of claim 1 further comprising: the stimulating mechanism isconnected to the reflecting surface and comprises a signal generator.90. The apparatus of claim 2 further comprising: the stimulatingmechanism is connected to the reflecting surface and comprises a signalgenerator.
 91. The apparatus of claim 3 further comprising: thestimulating mechanism is connected to the reflecting surface andcomprises a signal generator.
 92. The apparatus of claim 4 furthercomprising: the stimulating mechanism is connected to the reflectingsurface and comprises a signal generator.
 93. The apparatus of claim 5further comprising: the stimulating mechanism is connected to thereflecting surface and comprises a signal generator.
 94. The apparatusof claim 6 further comprising: the stimulating mechanism is connected tothe reflecting surface and comprises a signal generator.
 95. Theapparatus of claim 7 further comprising: the stimulating mechanism isconnected to the reflecting surface and comprises a signal generator.96. The apparatus of claim 8 further comprising: the stimulatingmechanism is connected to the reflecting surface and comprises a signalgenerator.
 97. The apparatus of claim 89 further comprising: thestimulating mechanism provides a signal that is essentially constantduring an ignition time and a high frequency alternating signal duringat least a portion of the time between the ignition time and asubsequent ignition time.
 98. The apparatus of claim 90 furthercomprising: the stimulating mechanism provides a signal that isessentially constant during an ignition time and a high frequencyalternating signal during at least a portion of the time between theignition time and a subsequent ignition time.
 99. The apparatus of claim91 further comprising: the stimulating mechanism provides a signal thatis essentially constant during an ignition time and a high frequencyalternating signal during at least a portion of the time between theignition time and a subsequent ignition time.
 100. The apparatus ofclaim 92 further comprising: the stimulating mechanism provides a signalthat is essentially constant during an ignition time and a highfrequency alternating signal during at least a portion of the timebetween the ignition time and a subsequent ignition time.
 101. Theapparatus of claim 93 further comprising: the stimulating mechanismprovides a signal that is essentially constant during an ignition timeand a high frequency alternating signal during at least a portion of thetime between the ignition time and a subsequent ignition time.
 102. Theapparatus of claim 94 further comprising: the stimulating mechanismprovides a signal that is essentially constant during an ignition timeand a high frequency alternating signal during at least a portion of thetime between the ignition time and a subsequent ignition time.
 103. Theapparatus of claim 95 further comprising: the stimulating mechanismprovides a signal that is essentially constant during an ignition timeand a high frequency alternating signal during at least a portion of thetime between the ignition time and a subsequent ignition time.
 104. Theapparatus of claim 96 further comprising: the stimulating mechanismprovides a signal that is essentially constant during an ignition timeand a high frequency alternating signal during at least a portion of thetime between the ignition time and a subsequent ignition time.
 105. Theapparatus of claim 97 further comprising: the stimulating mechanismcomprises a current generator that provides a first essentially constantdirect current during the ignition time and a second opposite polarityessentially constant direct current during the time between the ignitiontime and a subsequent ignitions time.
 106. The apparatus of claim 98further comprising: the stimulating mechanism comprises a currentgenerator that provides a first essentially constant direct currentduring the ignition time and a second opposite polarity essentiallyconstant direct current during the time between the ignition time and asubsequent ignitions time.
 107. The apparatus of claim 99 furthercomprising: the stimulating mechanism comprises a current generator thatprovides a first essentially constant direct current during the ignitiontime and a second opposite polarity essentially constant direct currentduring the time between the ignition time and a subsequent ignitionstime.
 108. The apparatus of claim 100 further comprising: the heaterelement comprises a current generator that provides a first essentiallyconstant direct current during the ignition time and a second oppositepolarity essentially constant direct current during the time between theignition time and a subsequent ignitions time.
 109. The apparatus ofclaim 101 further comprising: the stimulating mechanism comprises acurrent generator that provides a first essentially constant directcurrent during the ignition time and a second opposite polarityessentially constant direct current during the time between the ignitiontime and a subsequent ignitions time.
 110. The apparatus of claim 102further comprising: the stimulating mechanism comprises a currentgenerator that provides a first essentially constant direct currentduring the ignition time and a second opposite polarity essentiallyconstant direct current during the time between the ignition time and asubsequent ignitions time.
 111. The apparatus of claim 103 furthercomprising: the stimulating mechanism comprises a current generator thatprovides a first essentially constant direct current during the ignitiontime and a second opposite polarity essentially constant direct currentduring the time between the ignition time and a subsequent ignitionstime.
 112. The apparatus of claim 104 farther comprising: thestimulating mechanism comprises a current generator that provides afirst essentially constant direct current during the ignition time and asecond opposite polarity essentially constant direct current during thetime between the ignition time and a subsequent ignitions time.
 113. Amulti-layer reflecting coating forming an EUV reflective surfacecomprising: an inter-diffusion barrier layer comprising a carbideselected from the group SiC, ZrC and NbC.
 114. A multi-layer reflectingcoating forming an EUV reflective surface comprising: an inter-diffusionbarrier layer comprising a boride selected from the group ZrB₂ and NbB₂.115. A multi-layer reflecting coating forming an EUV reflective surfacecomprising: an inter-diffusion barrier layer comprising a disilicideselected from the group ZrSi₂ and NbSi₂.
 116. A multi-layer reflectingcoating forming an EUV reflective surface comprising: an inter-diffusionbarrier layer comprising a nitride selected from the group BN, ZrN, NbN,ScN and Si₃N₄.
 117. A multi-layer reflecting coating forming an EUVreflective surface comprising: a spectral filter tuned to selectivelyhighly reflect light in a band centered about at a first preferredwavelength and to significantly reduce the reflection of light at a bandcentered about a second wavelength.
 118. The apparatus of claim 117further comprising: the spectral filter comprises a plurality of nestedgrazing angle of incidence shells comprising reflective surfacescomprising the multi-layer reflective coating.
 119. An EUV light sourcecollector comprising: a plasma ignition point; a collecting mirrorhaving a focus at the plasma ignition point and comprising a normalangle of incidence multi-layer reflecting surface; a focusing spectralfilter comprising a plurality of nested grazing angle of incidenceshells comprising reflective surfaces comprising multi-layer grazingangle of incidence reflective surfaces.
 120. The apparatus of claim 119further comprising: the grazing angle of incidence reflective surfacesare selected to differentially reflect a first band of EUV light about afirst center wavelength and a second band of EUV light about a secondcenter wavelength within some range of grazing angle of incidence withinwhich the light from the collecting mirror is incident upon respectiveones of the plurality of shells.
 121. The apparatus of claim 119 furthercomprising: the collecting mirror comprises a spherical reflectingsurface.
 122. The apparatus of claim 120 further comprising: thecollecting mirror comprises a spherical reflecting surface.
 123. An EUVlight source collector comprising: a plasma ignition point: anelliptical collector mirror having a first focus at the plasma ignitionpoint and a second focus at an intermediate focus of the EUV lightsource; a debris shield intermediate the plasma ignition point and theelliptical collector mirror comprising a plurality of radially extendingchannels extending from the first focus and in symmetry about an axis ofrotation passing through the first focus and aligned to the longitudinalaxis of the elliptical collector mirror.
 124. The apparatus of claim 123further comprising: the plurality of channels are formed between aplurality of generally planer foils extending radially from the firstfocus and in symmetry about an axis of rotation passing through thefirst focus and aligned to the longitudinal axis of the collectormirror.
 125. The apparatus of claim 123 further comprising: theelliptical collecting mirror comprising an aperture centered on thelongitudinal axis of the elliptical collector mirror permittingirradiation of the plasma ignition point with a laser beam.
 126. Theapparatus of claim 124 further comprising: the elliptical collectingmirror comprising an aperture centered on the longitudinal axis of theelliptical collector mirror permitting irradiation of the plasmaignition point with a laser beam.
 127. The apparatus of claim 123further comprising: a plasma ignition point; a first collecting mirrorcomprising an elliptical reflecting surface having a first focus at theplasma ignition point; a second collecting mirror comprising a sectionof a spherical mirror having a center at the plasma ignition point anddisposed to collect light not striking the first collecting mirror andreflecting such light onto the first collecting mirror focused throughthe first focus of the first collecting mirror.
 128. A method ofreclaiming EUV light source collectors comprising a reflective surfacethat has become contaminated with debris comprising: opto-chemicallycleaning the collector reflective surfaces in a cleaning chambercontaining a carbon oxidizer containing gas and under irradiation froman ultraviolet light source.
 129. The method of claim 128 furthercomprising: the irradiating step is done with a light source irradiatingfrom essentially a point source at essentially the point source locationcorresponding the EUV light source plasma ignition point of thecollector in normal use.
 130. The apparatus of claim 128 furthercomprising: the ultraviolet light source is a DUV light source.
 131. Theapparatus of claim 129 further comprising: the ultraviolet light sourceis a DUV light source.
 132. A method of continuous removal of debrisfrom a collector reflecting surface in an EUV light source for theremoval of plasma produced residue debris on the reflecting surface,wherein the reflecting surface comprises a first material and theresidue debris comprises a second material comprising the steps of:creating a controlled sputtering ion source comprising the steps of:providing a gas comprising the atoms of the sputtering ion material;and, exciting the atoms of the sputtering ion material into an ionizedstate, the ionized state being selected to have a distribution around aselected energy peak that has a high probability of sputtering thesecond material and a very low probability of sputtering the firstmaterial.
 133. A method of continuous removal of debris from a collectorreflecting surface in an EUV light source for removal of plasma producedresidue debris on the reflecting surface, wherein the reflecting surfacecomprises a first material and the residue debris comprises a secondmaterial and compounds of the second material comprising the steps of:heating the reflecting surface to effectively remove residue debriscomprising the second material deposited on the reflecting surface; and,creating a controlled sputtering ion source comprising the steps of:providing a gas comprising the atoms of the sputtering ion material;and, exciting the atoms of the sputtering ion material into an ionizedstate, the ionized state being selected to have a distribution around aselected energy peak that has a high probability of sputtering thecompounds of the second material and a very low probability ofsputtering the first material.
 134. A method of continuous removal ofdebris from a collector reflecting surface in an EUV light source forremoval of plasma produced residue debris on the reflecting surface,wherein the reflecting surface comprises a first material and theresidue debris comprises a second material and compounds of the secondmaterial comprising the steps of: heating the reflecting surface toeffectively remove residue debris comprising the second materialdeposited on the reflecting surface; and, sputtering the compounds ofthe second material deposited on the reflecting surface using ions ofthe second material produced in the plasma.
 135. The apparatus of claim65 further comprising: the second material is a compound of lithium.136. The apparatus of claim 66 further comprising: the second materialis a compound of lithium.
 137. The apparatus of claim 67 furthercomprising: the second material is a compound of lithium.
 138. Theapparatus of claim 68 further comprising: the second material is acompound of lithium.
 139. The apparatus of claim 69 further comprising:the second material is a compound of lithium.
 140. The apparatus ofclaim 70 further comprising: the second material is a compound oflithium.
 141. The apparatus of claim 71 further comprising: the secondmaterial is a compound of lithium.
 142. The apparatus of claim 72further comprising: the second material is a compound of lithium. 143.The apparatus of claim 81 further comprising: the heater elementmaintains the temperature of the reflecting surface at a temperaturesufficiently high to evaporate the second material and low enough not todamage the reflecting surface materials.
 144. The apparatus of claim 82further comprising: the heater element maintains the temperature of thereflecting surface at a temperature sufficiently high to evaporate thesecond material and low enough not to damage the reflecting surfacematerials.
 145. The apparatus of claim 83 further comprising: the heaterelement maintains the temperature of the reflecting surface at atemperature sufficiently high to evaporate the second material and lowenough not to damage the reflecting surface materials.
 146. Theapparatus of claim 84 further comprising: the heater element maintainsthe temperature of the reflecting surface at a temperature sufficientlyhigh to evaporate the second material and low enough not to damage thereflecting surface materials.
 147. The apparatus of claim 85 furthercomprising: the heater element maintains the temperature of thereflecting surface at a temperature sufficiently high to evaporate thesecond material and low enough not to damage the reflecting surfacematerials.
 148. The apparatus of claim 86 further comprising: the heaterelement maintains the temperature of the reflecting surface at atemperature sufficiently high to evaporate the second material and lowenough not to damage the reflecting surface materials.
 149. Theapparatus of claim 87 further comprising: the heater element maintainsthe temperature of the reflecting surface at a temperature sufficientlyhigh to evaporate the second material and low enough not to damage thereflecting surface materials.
 150. The apparatus of claim 88 furthercomprising: the heater element maintains the temperature of thereflecting surface at a temperature sufficiently high to evaporate thesecond material and low enough not to damage the reflecting surfacematerials.
 151. The apparatus of claim 143 further comprising: thetemperature is between 400° C. and 700° C.
 152. The apparatus of claim144 further comprising: the temperature is between 400° C. and 700° C.153. The apparatus of claim 145 further comprising: the temperature isbetween 400° C. and 700° C.
 154. The apparatus of claim 146 furthercomprising: the temperature is between 400° C. and 700° C.
 155. Theapparatus of claim 147 further comprising: the temperature is between400° C. and 700° C.
 156. The apparatus of claim 148 further comprising:the temperature is between 400° C. and 700° C.
 157. The apparatus ofclaim 149 further comprising: the temperature is between 400° C. and700° C.
 158. The apparatus of claim 150 further comprising: thetemperature is between 400° C. and 700° C.
 159. The apparatus of claim151 further comprising: the temperature is between 450° C. and 650° C.160. The apparatus of claim 152 further comprising: the temperature isbetween 450° C. and 650° C.
 161. The apparatus of claim 153 furthercomprising: the temperature is between 450° C. and 650° C.
 162. Theapparatus of claim 154 further comprising: the temperature is between450° C. and 650° C.
 163. The apparatus of claim 155 further comprising:the temperature is between 450° C. and 650° C.
 164. The apparatus ofclaim 156 further comprising: the temperature is between 450° C. and650° C.
 165. The apparatus of claim 157 further comprising: thetemperature is between 450° C. and 650° C.
 166. The apparatus of claim158 further comprising: the temperature is between 450° C. and 650° C.